Zinc Phthalocyanine Thin Films Research Articles

Dielectric properties of zinc phthalocyanine thin films: effects of annealing in air and in N2

This work presents the effects of ambient conditions, in particular oxygen and humidity, on the dielectric spectra of thin zinc phthalocyanine (ZnPc) films equipped with interdigitated electrodes and the effect of annealing in dry N2 or in ambient air. The measurements were performed in the frequency range 10−2–105 Hz. The results indicate that the electric properties of ZnPc films are not only affected by oxygen but also by water vapour the presence of which always leads to the drop in alternating current conductance (ac-conductance).Moreover, at room temperature, the ac-conductance of ZnPc films previously exposed to air exhibits a reversible change with humidity, which makes these films attractive for humidity sensing applications.

Structural and optical properties of thermally evaporated zinc phthalocyanine thin films

Thermally evaporated zinc phthalocyanine (ZnPc) films in the as deposited condition were identified to be as-amorphous. It undergoes structural transformation upon annealing up to 613 K. The optical properties and spectral behavior of as deposited and annealed thin films of ZnPc were studied using spectrophotometric measurements of the transmissivity and reflectivity at normal incidence of light in the wavelength range 200–2500 nm. The refractive index, n , and absorption index, k , were calculated and it was found that they are independent of film thickness in the thickness range 205–530 nm. Annealing at 613 K increases absorbance of films by 5–6 times in comparison with absorbance of as deposited ones and shifts peak positions of all bands towards low energy side of spectra except the peak position of N-band is shifted towards high energy side of spectra. The absorption spectra in the UV–VIS. region has been analyzed in terms of both molecular orbital and band theories. Indirect allowed transitions near the onset and fundamental absorption edges were observed. The energy at the onset was obtained and equals to 1.45 and 1.51 eV for as deposited and annealed films, respectively. The fundamental energy gap was obtained and equals to 2.94 and 2.88 eV for as deposited and annealed films, respectively. The absorption spectra shows four absorption bands. The oscillator strength, f , the electric dipole strength, q 2 , the molar extinction coefficient, ζ molar , were calculated for as deposited and annealed ZnPc thin films.

Electrical conduction properties of flash evaporated ZincPhthalocyanine (ZnPc) thin films

Abstract Solar energy as the principle source has become very attractive, as it is abundantly available, can be tapped locally, pollution free and commercially viable over a period. Solar energy may be converted to other forms of higher-grade energy through one of several methods such as photothermal, photochemical, photoelectrochemical and photovoltaic (PV). Among these, as noted above the cleanest and most direct and efficient and of converts to electrical power is with the help of PV or solar cell devices. Photovoltaics are one of the hottest areas of research today. Nowadays organic material finds greater importance in the PV cell fabrications. Among the available organic materials ZincPhthalocyanine is a promising candidate for the solar cell applications, because of its easy synthesization and non-toxic to the environment. The major part of incident light in the visible region effectively contributes to photocarrier generation and the excited ZnPc molecules play an important role in PV effect. In this paper, we have reported electrical transport properties of flash evaporated ZincPhthalocyanine thin films have been reported. DC conduction mechanism in these films (Al–ZnPc–Al structure) was studied at different temperatures. The field dependence behaviour on activation energy and possible conduction mechanism in the ZnPc films under DC field has been discussed.

Thermally evaporated ZnPc thin films—band gap dependence on thickness

Abstract The ZincPhthalocyanine (ZnPc) thin films were obtained by thermal evaporation method. The X-ray diffraction analysis of vacuum evaporated ZnPc films reveals that the structure of the film is polycrystalline in nature for the samples grown at higher substrate temperatures. The crystallite size ( D ), dislocation density ( δ ) and strain ( e ) were calculated. The optical band gap energy has been calculated from the ( αh ν) 2 vs. h ν plot. The optical band gap energy of the ZnPc films was found to decrease with increase in film thickness.

Characterization of zinc phthalocyanine (ZnPc) for photovoltaic applications

Zinc phthalocyanine (ZnPc) is a promising candidate for solar-cell applications, because it is easily synthesized and is non-toxic to the environment. Recently, phthalocyanine (Pc) was considered by many researchers as the active part in all-organic solar cells, i.e. plastic solar cells. It is a self-assembling liquid crystal developed from a common deep-blue-green pigment. It exhibits a characteristic structural self-organization, which is reflected in an efficient energy migration in the form of extinction transport. In this paper we have report structural, surface morphological, optical and thermal properties of flash-evaporated zinc phthalocyanine thin films. The samples were prepared by using a vacuum coating unit on well-cleaned glass substrates under a pressure of 7×10-6 Torr. A constant rate of evaporation (1 A/s) was maintained throughout the evaporation of the ZnPc thin films. A rotary drive was employed to obtain uniform thickness during the evaporation. Thicknesses of the films were monitored by a quartz-crystal thickness monitor and were cross verified by the multiple-beam interferometry technique. The X-ray-diffraction pattern reveals the crystalline nature of the films deposited at higher substrate temperatures. Scanning electron microscope and scanning probe microscope nanoscope studies were carried out to determine the surface uniformity and homogeneity of the films for interfacing and application purposes. All the films were found to possess small crystallites less than 100 nm in size. The optical transmittance measurements were carried out using a spectrophotometer in the visible region (400–800 nm) and the films were found to be absorbing in nature. The band gap of the ZnPc thin films is 1.97 eV and the optical transition was found to be direct and allowed. The absorption coefficient, extinction coefficient and refractive index of the ZnPc films were evaluated and the results are discussed. Differential scanning calorimetry studies of ZnPc films were carried out and a phase transition from α to β was observed at 538 K.

Effects of water in phthalocyanine layers

The molecular water concentration inside zinc phthalocyanine (ZnPc) thin films was measured. After exposure to air, gas effusion experiments show that the ZnPc layers contain (1.7±0.4)×10 20 water molecules per cm 3, which corresponds to 1 H 2O per 10 ZnPc units. We can distinguish a mobile and an immobilized population of H 2O in ZnPc films. The mobile part effuses out at room temperature when exposing the films to a low pressure of 10 −2 mbar, whereas temperature activation is needed to reach a complete out-diffusion of water. The effusion process was observed to proceed with a diffusion coefficient D H 2O of (1.3±0.3)×10 −10 cm 2 s −1 at 296 K. The rate of water effusion directly correlates with the timescale of the decrease of surface conductivity when exposing the layers to an equally low pressure. This indicates the existence of an electrically active surface layer of water molecules, which is refilled from the bulk of water molecules during the effusion process.

Space charge analysis in doped zinc phthalocyanine thin films

We present an improved method for the determination of the space charge density in organic semiconductors used as active layers in Schottky barriers. These measurements provide a powerful tool for the interpretation of basic properties such as the rectifying effect, doping process and carrier trapping mechanisms of films together with a way to assess the potential for sensor applications. Metal/molecular semiconductor Schottky junctions were prepared on zinc phthalocyanine layers doped by a controlled exposure to the ambient air. The organic material is deposited on aluminium or heavily doped silicon substrates, in order to make a Schottky barrier (film thickness around 1 μm). An ohmic contact is obtained by a gold deposition on the strongly doped side of the molecular material. We have investigated the current-voltage and capacitance-voltage characteristics. The results are interpreted in terms of a space charge region at the interface with the substrate, followed by an extended semi-insulating layer.The contribution of these two regions to the total impedance is analyzed in well improved conditions of measurements.

Influence of iodine on the electrical and photoelectrical properties of zinc phthalocyanine thin film devices

The electrical and photovoltaic properties of the zinc phthalocyanine (ZnPc) and I 2 doped ZnPc thin films, sandwiched between indium tin oxide (ITO) and Al electrodes, were investigated. Doping with iodine brings adequate changes in the characteristics of the device. The devices constitute a metal-insulator-semiconductor (MIS) structure, in which depletion layer is formed in ZnPc, near Al-Al 2O 3/ZnPc. The depletion layer width and potential barrier height decrease with I 2 doping. The charge transport phenomenon at higher voltage range appears to be space charge limited conduction (SCLC), in the presence of the discrete trapping level. The position of Fermi level shifts toward the valence band edge, which indicates that I 2 doping increases the P-type conductivity. Various electrical and photovoltaic parameters were determined from the J-V and C-V analysis. The influence of the I 2 doping has been discussed in detail.

Electrical, structural and gas sensing properties of zinc phthalocyanine thin films

The dark conductivity of zinc phthalocyanine (ZnPc) has been studied as a function of material purity, crystal phase transformation and temperature with particular regard to gas sensitivity in air, O 2, N 2, argon, NH 3 and NH 3-air mixtures. α-ZnPc was found to grow in the form of randomly oriented micro- crystallites but the β form showed oriented needle-like and whisker growth. The electrical properties were found to be dependent on material purity. Entrainer- sublimed ZnPc showed higher conductivity than impure material and displayed reproducible linear characteristics with less drift and hysteresis. The conductivity of both α- and β-ZnPc is found to be critically dependent on the presence of O 2. The sensitivity to other gases differs for the α and β forms but in both cases NH 3 causes a large dark conductivity decrease, possibly owing to catalytic behaviour, effectively removing oxygen acceptors. Conductivity-temperature data indicate a transition from extrinsic to non-extrinsic conduction for most cases. The conductivity of β-ZnPc is found to be greater than that of α-ZnPc, in contrast with other phthalocyanines. The relative sensitivities to the various gases suggests that ZnPc may be a viable material for selective gas sensing devices.

Oxygen doping of zinc phthalocyanine thin films

It is well established that the electrical properties of phthlocyanine solids change drastically as these solids absorb oxygen. However, the actual doping process has not been previously investigated. Here, capacitance measurements of (Al‖zinc phthalocyanine‖Nesatron) cells are reported which show the formation of a Schottky barrier at the Al‖zinc phthalocyanine interface as a function of distance from the interface and as a function of exposure time of the cell to oxygen. A careful examination of aged zinc phthalocyanine Schottky cells shows an inhomogeneous space charge density close to the aluminum contact. This inhomogeneity could lead to erroneous determinations of the barrier potential.